1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) apparatus.
2. Description of the Prior Art
A CT apparatus is described in U.S. Pat. No. 4,637,040 which has an X-ray source which is rotated around an examination subject for obtaining datasets (projections) from a number of different projection angles, the X-ray source having a focus which is moved back and forth between two end positions (alternating focus). A detector system, composed of a number of detector elements, receives the X-rays emanating from the alternating focus and attenuated by the examination subject. Successive, adjacent detector elements which are disposed at a grid element spacing from an adjacent detector element, each detector element having an aperture. The detector elements in the aperture emit output data, corresponding to the aforementioned attenuated X-rays, the number of output data items corresponding to the number of detector elements which participate in obtaining the data for a given projection. The output data are supplied to a computer which reconstructs an image of the examination subject, or a portion thereof, based on the output data.
In a CT apparatus such as this, besides the size of the focus of the X-ray source, the MTF (Modulation Transfer Function) of the reconstruction algorithm and the pixel size of the reconstructed image, the scanning frequency with which the projection set is obtained, and thus the grid element spacing between immediately adjacent detector elements, are important determinants of the achievable spatial resolution.
The movement of the focus between two end positions, that is, the use of an X-ray tube with an alternating focus, serves to enhance the spatial resolution, by increasing the scanning frequency, and is taught by the above cited U.S. Pat. No. 4,637,040.
In addition, U.S. Pat. No. 4,008,400 to increase the scanning frequency by obtaining complementary projection datasets by what is known as the .lambda./4 shift. However, the .lambda./4 shift presumes a CT-device with highly stable and precise mechanical relations and is also not very effective when the path radius at which the focus of the X-ray source moves around the subject is short.